Oxidation catalyst

ABSTRACT

The present invention relates to a process for preparing acrylic acid by oxidizing acrolein and to an oxidation catalyst. More particularly, the present invention relates to a catalyst and a process for preparing acrylic acid by oxidizing acrolein or a mixed acrolein gas obtained by vapor phase oxidation of propylene with molecular oxygen in the presence of an oxidation catalyst of the empirical formula:

United States Patent [191 Takenaka et al.

[ Dec. 31, 1974 OXIDATION CATALYST [75] Inventors: Shigeo Takenaka;Hitoshi Shimizu; Masanobu Ogawa, all of Takasaki, Japan [73] Assignee:Nippon Kayaku Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Feb. 26, 1973 [21] App]. No.2 336,058

[30] Foreign Application Priority Data Mar. 9, 1972 Japan 47-23603 [52]U.S. Cl 252/467, 252/456, 260/530 N [51] Int. Cl B0lf 11/06 [58] Fieldof Search 252/456, 467; 260/530 N [56] References Cited UNITED STATESPATENTS 3,567,773 3/1971 Yamaguchi et a1 ..252/467X PrimaryExaminer-Daniel E. Wyman Assistant Examiner-W. J. Shine Attorney, Agent,or Firm-Russell & Nields [5 7] ABSTRACT The present invention relates toa process for preparing acrylic acid by oxidizing acrolein and to anoxidation catalyst.

More particularly, the present invention relates to a catalyst and aprocess for preparing acrylic acid by oxidizing acrolein or a mixedacrolein gas obtained by vapor phase oxidation of propylene withmolecular oxygen in the presence of an oxidation catalyst of theempirical formula:

Mo a b c d e 2 Claims, No Drawings OXIDATION CATALYST BACKGROUND OF THEINVENTION Acrolein may be obtained by oxidizing propylene. In thepreparation of acrylic acid by vapor phase catalytic oxidation ofpropylene, there have been the following three manners of carrying outthe reaction:

1. One-step oxidation process wherein propylene is oxidized directlyinto acrylic acid in the presence of a catalyst.

2. Two-step process wherein acrolein is mainly prepared in the firstoxidation step, acrolein is then separated from byproducts such asacrylic acid and acrolein is oxidized in the second oxidation step. 3. Aprocess wherein acrolein is mainly prepared in the first oxidation stepand oxidized in the second oxidation step without separating acroleinfrom by-products or off gas (hereinafter this process will be referredto as the continuous process).

The present invention can be applied to either process (2) or (3).Particularly, if the present invention is applied to the continuousprocess, the merit that the yield of by-produced propionic acid can bereduced while the high yield of acrylic acid is kept can be obtainedpositively. Though acrylic acid obtained by vapor phase catalyticoxidation of propylene contains a small amount of propionic acid,separation of propionic acid therefrom is quite difficult by either aphysical process such as distillation or by a chemical treatment, sincethey have nearly the same physical properties (such as boiling point)because the molecular weight of propionic acid is very close to that ofacrylic acid, and since their chemical properties are very close to eachother because their chemical structures are not so different from eachother.

Further, the presence of propionic acid or an ester thereof which cannotbe polymerized exerts a great influence upon the quality of acrylic acidor an ester -thereof.

After investigations of reducing the by-production of propionic acid inthe oxidation step, the inventors have confirmed that propyleneunreacted in the first oxidation step and contained in the off gas iscatalytically oxidized into propionic acid in the second oxidation stepon a catalyst mainly comprising molybdenum and vanadium. Further a verysmall amount of propylene is converted into propionaldehyde in the firstoxidation step and then propionaldehyde is converted into propionic acidin the second oxidation step.

Molybdenum-vanadium catalysts used for the preparation of acrylic acidfrom acrolein have been previpropylene by the continuous process, if100% of propylene is oxidized in the first oxidation step, propionicacid prepared will be in only a very small amount. However, 100%conversion of propylene causes a great reduction in selectivity toacrolein. Therefore, for preventing reduction in single pass yield ofacrolein, conversion of propylene must be controlled to about 95 97%.Consequestly, 3 5% of propylene is introduced in the second oxidationstep in this case.

DETAILED DESCRIPTION The inventors considered whether the catalystmainly comprising molybdenum and vanadium for the second oxidation stepcould'be inactivated against propylene, keeping activity thereof againstacrolein, by partial poisoning of the catalyst. If active points of thecatalyst against acrolein and propylene are the same, this technique isimpossible but if the points are different, this technique may bepossible. On the basis of this consid eration, the inventors have madeexperiments by using various poisons and found that the purpose can beattained by the addition of a small amount of an alkali metal.

Among the alkali metals, sodium is the most effective. Other alkalimetals, that is potassium, lithium, rubidium and cesium, are effectivetoo. If the amount of the alkali metal is too large, the activityagainst acrolein is also lost and, on the other hand, if the amount istoo small, the effects of poisoning activity against propylene isreduced. The catalyst of the present invention and the same catalyst butcontaining no alkali metal component were prepared in the same mannerand they were used for the reaction of the same gas from the firstoxidation step under the same conditions. In the presence of thecatalyst of the present invention, propionic acid content was 550 ppm(mole) based on acrylic acid, while in the presence of the catalystcontaining no alkali metal component, propionic acid content was l,300ppm. Thus, by using the catalyst of the present invention, propionicacid content was reduced remarkably, while no great difference wasobserved in yield of acrylic acid (a little over molar based onpropylene in both cases).

The catalyst of the present invention has the empirical formula:

Mo V T A O wherein Mo, V and 0 represent molybdenum, vanadium andoxygen, respectively, T represents tungsten or antimony, A represents analkali metal, and a, b, c, d and e represent number of atoms of Mo, V,T, A and 0, respectively, and when a is 12, b is 0.5 m6, c is 0 to 6, dis 0.01 to 1.5, preferably, 0.15 to 1.2 and e is a number naturallydetermined by the valence requirements of the other elements present.

The catalyst of the present invention is prepared by known method, forexample, by mixing a water-soluble molybdate such as ammonium molybdate,a watersoluble vanadate such as ammonium vanadate, a watersoluble saltof an alkali metal such as a sodium nitrate and, optionally, tungsten orantimony compound all in the form of aqueous solution or powder, addingif necessary, a suitable carrier such as Aerosil (finely powdered silicagel; a trade name of Degussa), evaporating the mixture to dryness,pulverizing the resulting cake into pieces having a suitable size andcalcining them at a temperature of from 300 to 500 C, preferably from350 to 450 C, in the presence of oxygen.

The starting materials used to prepare the catalyst such as molybdate isnot restricted to the above mentioned compound.

Any starting material which can form the metal oxide or complex metaloxide with other metal after calcination treatment is useful.

Suitable carrier include silica, silicon carbide, alu- 4 from the firstoxidation step. 83 of the reacted propy lene is converted into acroleinand 6 is converted into by-produced acrylic acid. The rest comprisesmainly carbon dioxide and carbon monoxide and a mina, diatomaceousearth, titanium oxide, etc. The catsmall quantity of acetic acid is alsocontained therein. alyst is used in the form of granules or aftershaping The gas exhausted from the first oxidation step cominto tabletsin a fixed bed or it may be used in the form prising these gases,residual oxygen, nitrogen which of small particles in a fluidized bed ormoving bed. does not participate in the reaction and steam is di- Whenthe continuous process is adopted, a catalyst rectly introduced in thesecond oxidation step. used in the first oxidation step may be anycatalyst 10 A catalyst used in the second oxidation step is prewhichoxidizes propylene into mainly acrolein. pared in the following manner.

In the first oxidation step, propylene is introduced to- 66.1 g ofammonium paramolybda g of gether with air or mixed oxygen gas. Ifnecessary, steam (hum "mate and g of ammohlum methavahadate may l b i dd h i Th gas mixture Sent all dissolved in distilled water are mixedtogether. Furfrom the first oxidation step to the second oxidation ther,40 g of AeTOSll h which z mitten! is 21 are step comprises acroleinproduced in the first oxidation q f thereto as Camer- The Whole l5heated under Step unreacted propylene, Oxygen, nitrogen, stirring andevaporated to dryness. Then, the produced produced acrylic acid, aceticacid, carbon monoxide, Cake 15 Plllveflled t0 5 m SIZE and calcined atcarbon dioxide and steam 400 C in air stream for 4 hours.

The gas mixture is passed on the catalyst of the sec- 20 Composition ofthe catalyst thus obtained is repreond oxidation step and the contacttime is usually from sented by the formula:

0.5 to 10 seconds (NTP). The catalytic oxidation process of the secondoxidation step can be carried out at MolzvsNaMO (Gamer S102) atemperature of from 250 to 350 C and at a pressure 177 ml of thecatalyst are placed in a stainless steel tube of-from 0.5 to 10atmospheres. 25 of 20 mm inside diameter immersed in a molten potas- Themo] ratios ofingredients in the gaseous feed mIX- sium nitrate bath,into which the gas containing acrotllre t0 the Second oxldatloh p P yare from lein from the first oxidation step is introduced. Temper- 0.5to 5 mols of oxygen and from 1 to 20 mols ofsteam ature f h b ifregulated to 2709 R h of P Th6 Process of h P mveh' 0 Single pass yieldof the acrylic acid is 71.0 Unretion 18 further illustrated by thefollowing examples. In 3 acted propylene is 43 and by pmduced acroleinis the present specification, the following definitions are ,3 theremainder being composed mainly of Carbon P l' I dioxide, carbonmonoxide and acetic acid.

Single pass yield of acrylic acid Mols of acrylic propionic acid Contentis 550 ppm.

acid obtained/Mols of propylene fed X 100 Propionic acid content (ppm)Mols of propionic COMPARATIVE EXAMPLE 1 6 acld Qbtamed/Mols of acrylicacld obtamed X 10 The first oxidation step is carried out in the sameEXAMPLE 1 manner as in Example 1 with respect to the apparatus,catalytic reaction conditions, etc. The gas exhausted Catalyst of l ioxldauon step is Prepared by a 40 from the first oxidation step isintroduced in the second method described in Example 1 of JapanesePatent oxidation step. The second oxidation step is the same PublicationNo. 6245/1969. The resulting catalyst has as in Example 1 except thatthe same catalyst as m the the following general composition.

second oxidation step of Example 1 but containing no 4.5 4 i 1 6.os 1z52 Na [general composition: Mo V O (carrier SiO is 140 ml of thecatalyst are placed in a stainless steel 45 lreaction tube of 20 mminside diameter. The reaction Yleld of the acrylc acld hnleacted P PY'tube is immersed in a 330 C of molten potassium nilene h Y'P acrolelh l53 trate bath and a gas mixture comprising propylene, air PFOPIQY"c acldContent 1,300 PP and steam in molar ratio of l:l2:6. is introducedtherein, the contact time being 6 seconds (NTP). EXAMPLES 2 95.0 ofpropylene introduced in the first oxidation The procedures described inExample 1 are repeated step has been reacted and the remainder 5 isconexcept that the alkali metal content and/or kind of altalne i asunreacted propylene in the ga x aus kali it is changed The results areshown-in Table 1.

Table 1 Catalyst Reaction Single pass Propionic Example composition bathtemyield of acid conperature acrylic acid tent (C) ("/r) (ppm) 2M6,,V,Li,, ,0,, 270 70.5 670 3 Mo vnt o 270 70.7 590 4 Mo V Na O 27071.0 600 5 i2 o.2s 4,1 2 0 71.2 880 6 Mo V Na ,0, 270 66.5 500 7 M,,v,Rb,, ,o, 270 69.5 680 8 ivio v,cs,, .,o, 270 69.0 690 Emma The firstoxidation step is carried out in the same procedure as in Example 1. Acatalyst prepared as follows is used in the second oxidation step. 66.1g of ammo nium paramolybdate, 10.9 g of ammonium methavanadate, 9.5 gofammonium paratungstate and 1.3 g of sodium nitrate all dissolved indistilled water are mixed together and the mixture is further mixed with40 g of Aerosil. The mixture is evaporated to dryness under stirring.Then, the produced cake is pulverized into 5 20 mesh size and calcinedat 400 C for 4 hours.

Composition of the catalyst thus obtained is represented by the formula:

The catalyst is used in the second oxidation step in the same manner asin Example 1. The same reaction conditions as in Example 1 are employed.

Single pass yield of acrylic acid is 72.0 Propionic acid content ofresulting acrylic acid is 570 ppm.

COMPARATIVE EXAMPLE 3 Mo V Sb O (carrier SiO 4 hours. Composition of thecatalyst thus obtained is represented by the formula:

Mo V Sb N a O (carrier SiO Mo V W Na 0 (carrier SiO l5 I 1 0 h Singlepass yield of acrylic acid is 73.1 Propionic T 18 cat a ys t 18 use as te 0X1 8110 11 cata ystm t e acid content is 1,300 ppm. second oxidationstep. The same reaction conditions as in Example 1 are employed. Singlepass yield of acrylic acid is 73.3 Propionic acid content is 570 ppm.EXAMPLES 11 7 21 A The first oxidation step is carried out in the same AP E COMPARATIVE EX M L 2 manner as in Example 1. Catalysts shown inTable 2 The Procedure described Example 7 f repeatefi prepared in thesame manner as in Example 7 or Examexcept that the catalyst having thefollowing composipig 8 were used in the Second Oxidation Step 15 used mthe Second Oxldatlon Step' Reaction conditions, except for reaction bathtem- M012V3w 2O41 (carrier SiO perature, and reaction apparatus are thesame as in Ex- Single pass yield of acrylic acid is 75.2 Propionic ampleacid content is 1,300 ppm. The results are shown in Table 2.

Table 2 Catalyst Reaction Single pass Propionic Example composition bathtemyield of acid con perature acrylic acid tent (C1 (7:) (ppm) 11Mo,,v,w, ,Na,o,, 270 70.2 520 12 Mo, V, ,W K ,,0,, 280 72.1 540 13Mo,,v,w,r-1a,, ,o, 270 72.8 560 14 Mo, V W,Li,O,, 270 71.9 610 15Mu,,v.,w.,,|.i., ,o,, 280 69.8 690 16 Mo,,v,sb,| ,o,, 270 70.1 600 17Mo, V Sb Na O 270. 71.9 590 18 Mo,,v., ,w., ,Na,o,, 280 68.0 530 19M0.,v,v,sb.,Li,,,o 280 68.5 580 20 Mo,,v,sb,Na., .,,o,, 270 67.5 900 21i2 2.s u.s i.s -|ss 270 6 .0 300 (carrier SiO EXAMPLE 10 We clam I l. Acatalyst represented by the empirical formula, The first oxidation stepis carried out in the same procedure as in Example 1. A catalystprepared as follows a b c d e 15 used in thelsggond logigatiofn step.66.1 g ofhamm wherein, Mo, V and 0 represent molybdenum, vanamum paramoy a 0 P f j dium and oxygen, respectively, T represents tungsten dateand 1.3 g of sodium nitrate all dissolved in distilled or antimony, Arepresents an alkali metal, and a, b, c, water are mixed together andthe mixture 15 further d re r em b rof t fM V T A d mixed with 14.2 g ofantimony trioxide powder and 40 8 3' a :2 2 8 f g of Aerosil. Then themixture is heated under stirring respFc We an w an r to t C andevaporated to dryness. Thus obtained cake is pul- 1 to and e a numbernaturally deter verized to 5 -20 mesh size and calcined at 400 C formmed y the Valence requirements of the other ments present.

2. The catalyst of claim 1, wherein a is 12, b is 0.5 to 6, c is 0 to 6,d is 0.15 to 1.2.

1. A CATALYST REPRESENTED BY THE EMPIRICAL FORMULA,
 2. The catalyst ofclaim 1, wherein a is 12, b is 0.5 to 6, c is 0 to 6, d is 0.15 to 1.2.